In the transport and energy sectors, many thermal equipment items include hot parts. There are for example boilers, ovens, blast furnaces, stationary gas turbines, jet engines, turbochargers, or internal combustion engines. The “hot path” of this equipment is defined as the set of volumes and pipes in which the stream of gas and particles flows. As the lifetime and the performances of the hot parts strongly depend on their interactions with the stream, it is important to be able to predict these interactions as accurately as possible.
Ash particles have three possible origins. They can come from a fuel containing various foreign elements which will be designated by the term “contaminants,” namely: alkali, alkaline-earths or transition metals; sulfur; aluminum; silicon; chlorine; phosphorus; mercury; etc. These “contaminated” fuels, of fossil or biogenic origin, can be solids (coal; lignite; wood; straw; peat; domestic residue; etc.), liquids (contaminated petroleum distillates; heavy fuels; HCO; LCO; oil residues; raw vegetable oils; alcohols; bagasse; biodiesel; etc.) or gases (synthesis gas or “syngas;” blast furnace or coke-oven gas; biogas; etc.). Ash can also come from the combustion air: for example, jet engines are fed with fuels of high purity (kerosene; jet fuel) but they ingest substantial quantities of contaminants contained in the atmosphere, such as sea salts or dust, in particular particles of “CMAS” (“calcium magnesium aluminosilicates”), which undergo some transformations while crossing the flames. Finally, they can come from “ash modifying agents” or “ash modifiers,” that is to say substances that are introduced intentionally into the combustion systems to inhibit the corrosion of the hot parts or to modify the nature and the properties of the inorganic phases formed in the flames.
Whatever their origin, there are three steps in the fate of ash particles: a step of generation or transformation in the flame; a step of transport within combustion gases; possibly one or more shock(s) with machine components. After the step of generation or transformation, the ash particles are in more or less stable suspension within the stream of combustion gases (“carrier gas”) and cross the hot path (step of transport) in a solid, liquid, or partially liquid/solid state, according to the local thermal conditions.
One is then in the presence of a dual stream. One stream is a gas stream which forms, according to the terminology of fluid dynamics, a “stream tube” and which is described by four main parameters: geometrically by its “section” which is the cross-section of the stream tube, kinematically by a speed field, thermally by a temperature field, and chemically by its composition (O2; CO2; H2O; SO2 etc.). The other stream is a particles stream that moves in this same “stream tube” and has the same speed, temperature, and cross-section as the gas stream, and is also defined by the concentrations of the phases contained in the particles.
The collision between this dual stream and a target generates, from the point of view of the stream, multiple kinematic effects: deviation of the gas trajectory with the creation of eddies; rebounds of certain particles on the target with changes of their trajectories; “capture” of some particles by the target, possibly followed by their expulsion by other particles. It also generates complex physicochemical “interactions” between the stream of gas and particles and the target, which cover in particular: matter transfers (formation of deposits or scratching-out of material from the target); heat transfers (by forced convection); mechanical effects (shocks undergone by the part); chemical interaction (possible reactions between the target wall and the particles and/or gases: oxidation; sulfidation; etc.); metallurgical effects (superficial hardening of the target; phase changes in the thermally affected zone, etc.). These interactions determine the main three “modes of degradation,” which the material of the target can undergo, namely: erosion; fouling, and corrosion.
Erosion is a physical degradation of the material surface caused by high impact speeds and some hardness of the particles; it affects the lifetime of the hot parts. Fouling is a form of reversible or irreversible degradation of the surface cleanliness of the material which results from the formation of notable quantities of deposits having properties of adhesion to the target and/or aggregation and internal cohesion. Fouling has a negative impact on the aerothermodynamics performances of the hot parts and, when it is irreversible, on their lifetime. In the following, the term “deposition” will be used to indicate the formation process of a deposit. One can define the “deposition rate” on a portion of surface of the target as the mass of particles which settles on it, divided by the time and the surface area. It is noteworthy that, when the particles are in the liquid or in a pasty state, i.e., are at a temperature higher than their solidus, their deposition rates are much higher than the same particles taken in the solid state. Moreover, an initial process of fouling consisting of the deposition of an ash film can be followed by the formation of an additional corrosion film (by hydrated iron oxides, for example).
In the third damaging mode which is corrosion, the degradation of material results from a chemical and/or electrochemical attack by the particles and/or the gas. The lifetime of the hot parts is also impacted by corrosion. For corrosion caused by ash, it is necessary that either corrosion starts from a previously formed deposit, which can be very thin, not visually detectable and not fouling, or corrosion is coupled with erosion (effect of “erosion-corrosion”). Consequently, it is rational to distinguish on the one hand two modes of “primary interactions” which are erosion and deposition, and on the other hand a mode of “secondary interaction”—corrosion—which can be associated with one or the other of the primary interactions.
As a consequence of the very fast kinetics of electrochemical attacks in molten electrolytes, a molten phase will cause corrosion with much higher severity than a solid phase. Two modes of degradation can combine and have harmful synergistic effects as in the case of erosion-corrosion and corrosion under deposit. In stationary or aircraft gas turbines, the three modes of degradation can be encountered in a differentiated manner according to the conditions. For example, particles of CMAS in the molten form (high temperature) will tend to foul then to corrode, while the same particles present in the solid state will rather tend to erode the hot parts. One will thus note the complexity resulting from such combinations between the modes of interaction. Therefore, to define sure strategies of prevention or reduction of these effects, it is required for expensive installations, or for equipment subject to drastic safety regulations (stationary gas turbines; jet engines), it is necessary to resort to experimentation to reproduce the feared mode(s) of degradation. Such experimentation must be representative of the target and the stream of gas and particles. Now, it is certainly easy to reproduce the properties of the target that influence the step of interaction. These properties are primarily its geometry, the chemical composition and the metallurgical structure of the material, its mechanical and thermal properties, and its surface quality which can result from a mechanical treatment (polishing, sand-blasting, shot-peening, etc.) or from a metallic, ceramic or “cermet” type coating. On another hand, it is more difficult to reproduce a “representative stream,” i.e., to reproduce all the characteristics of the gas stream and especially those of the particle stream intervening in the step of interaction. These characteristics are (i) for the gas stream: the temperature and speed fields, the degree of turbulence, the gas composition; and (ii) for the particles stream: the kinematic characteristics (with speeds being identical to those of gas); geometrical properties (sizes); thermal properties with temperatures being identical to those of gases; coefficient of expansion; conductibility; mechanic properties (hardness; elasticity modulus; impact resistance); physical characteristics (melting point; crystallize, amorphous or vitreous state; porosity; rheology in the case of liquids) and chemical properties (reactivity with respect to the material of the target). It is clear that most of these characteristics cannot be reproduced without re-creating the actual source of particles.
It is appropriate moreover that one also controls the duration of the collision and the local collision conditions, i.e., the conditions which prevail at the very point of impact and which include geometric conditions such as the angle of incidence; aero-thermal conditions such as the speed and temperature of gas and particles; the skin temperature of the target which depends not only on the gas temperature but also on the thermal losses of the target as it will be specified below.
One will speak about “collision in controlled conditions” when all the above mentioned conditions are controlled. However, a review of prior art on the subject shows that the existing processes and experimentation devices do not meet these criteria of representativeness or suffer from shortfalls or major drawbacks. A first traditional experimental method consists of maintaining a target within a “bed” of real or synthetic ash powder, for a defined duration, under a controlled atmosphere. This method, which often is called “immersion test,” proceeds in isothermal conditions. It is primarily static, because, even if the atmosphere can be put in circulation, the sample is not exposed to a stream of particles but is immersed in a bed of static particles and is not in direct contact with the gas.
A second traditional method, often called a “thermo balance test,” consists of forming an initial deposit of ash on the target, for example by spraying a fog of a solution of the “precursors” and then passing it in a flame. The term “precursors” designates substances which generate, at high temperature, the particles and possibly certain components of the gas stream such as SO2. The target is then introduced into a thermo balance within which the temperature and the composition of the atmosphere are controlled. One can thus monitor, in isothermal conditions, the ash/material interaction by thermo-gravimetric analysis. However, the interaction between the particles and the target are also static here since the deposit preexists when the test starts. In fact, because of the design and the risks of fouling/corroding the “noble” components of the thermo balance, one cannot make particles circulate in it during the test.
Consequently, both the “immersion test” and “thermo balance test” are primarily static in nature and reproduce neither the process of continuous generation of a deposit, nor the velocity of particles.
A third method relies on the use of “burner rigs” which are mainly intended to study corrosion at high temperature. According to a typical design of a burner rig, several probes of the target are placed in an isothermal oven which is swept by the combustion gas stream produced by a burner. To generate the stream of ash particles, one installs, in the burner/oven connection duct, an injector that is fed with an aqueous solution containing the desired precursors. One can extract at defined dates the coupons from the oven, quantify the deposits formed thereon, and subject them to chemical, metallographic, and mechanical tests.
These traditional burner rigs suffer from several limitations and disadvantages. A first major disadvantage lies in the basically static character of these tests and the impossibility of imposing any important and fast temperature variation due to the strong thermal inertia of the rig which is necessary for obtaining a good temperature control. A second disadvantage lies in the relatively low speed range (from a few cm/s to a few m/s) that one can create in it in an economic way due to the non-negligible sections of the various elements of the hot path (conducts and oven). Let us consider for example, a miniaturized burner rig in which the oven would have a section of passage of only 10×10 cm (hardly allowing the handling of the probes). A simple calculation shows that it would be necessary, to obtain a temperature of 850° C. and a speed of 300 m/s, to generate a combustion gas flow of approximately 10,000 m3/h and to burn approximately 130 l/h of fuel (kerosene or diesel fuel). Moreover, the strong pressure drop caused by this high speed would require using an air compressor to feed the burner and a design of the hot path acceptable at the temperature and the pressure (tightness constraints). The generation of high speeds would thus induce elevated investment and operation costs. A third disadvantage is related to the poor definition of the stream lines of the gas and particles inside the oven, precisely because of the limited speeds, with a risk of stratification or even segregation of the particles at the bottom of the oven by gravity effect. A fourth disadvantage lies in the possible chemical interferences between certain anions and cations in the aqueous solution to be sprayed. For instance, one cannot mix calcium or barium ions with sulfate ions which are used as SO2 precursors. Finally, a fifth disadvantage lies in the existence of “memory effects.” During a given test “E,” one of the injected precursors (e.g., X) can partially be retained on the walls of the hot path (by deposition, adsorption, or absorption) and be then released during a later test “E+n” especially if this test is carried out at higher temperature and speed. Such “a memory effect” that tends to distort both test “E” (defect of X) and test “E+n” (undesirable presence of X) is also insidious as it is detected only during the test “E+n.” Such effects are encountered for example with chromium oxide or boron oxide. The closer to the oven the precursors are injected, the less they are likely to be partially retained. One can thus inject the solution in a point close to the entry of the oven containing the probes. However this does not suppress the risk of retention on the walls of the oven. As this effect affects insulating materials both at their surface and in their bulk, the only way of removing this risk is to frequently change these insulating materials, which results in a costly and tedious procedure when one wishes to carry out programs comprising a significant number of tests with very different ash compositions.
Consequently, the existing methods either do not make it possible to create streams representative of gas and particles at high temperature and high speed, or present major drawbacks. It will be noted that these three methods rely on two common designs points. First, the gas stream is entirely confined in a tight and isothermal set of conduct and oven. Second, the target is entirely contained in this oven and is thus exposed to rigorously isothermal conditions in all its points. These design points aim at creating isothermal and uniform conditions of interaction for the target, conditions which are regarded as essential in order to obtain sufficiently repeatable and reproducible results.
The present invention aims at remedying these disadvantages of the existing processes, in particular those related to the thermal inertia.